At present, a mortar multi-wire cutting technology is mainly used to cut crystalline silicon slices used in the photovoltaic industry. However, this technology has many problems such as low cutting efficiency, high cost and waste mortar emission pollution after cutting. In contrast, a method for slicing polycrystalline silicon ingots by diamond wires has attracted much attention because of the advantages of low processing cost, high cutting efficiency and environment friendliness. However, compared with a mortar cut silicon slice, a diamond wire cut polycrystalline silicon slice has a thinner surface of damaged layer, and reactivity is insufficient. A traditional acid texturing process used to cut polycrystalline silicon slices with mortar cannot achieve texturing of diamond wire cut polycrystalline silicon slices. People in the crystalline silicon battery industry have been eager to solve this problem.
The invention entitled “Texturing method for diamond wire cut polycrystalline silicon slice” with the application number 201410430817.2 discloses that texturing is carried out by using an alkaline mixed solution containing sodium hydroxide, absolute ethanol or isopropanol, and a textured surface prepared by the method has a good light trapping effect and can make the surface reflectivity of the silicon slice lower. However, due to the fact that the surface of the polycrystalline silicon slice has various crystal planes such as [100], [110] and [111] which have different reactivities, a uniform textured surface with small color difference and high light conversion efficiency is hard to form with the alkaline solution, and a fill factor of a manufactured battery piece is also high; furthermore, according to the method, texturing needs to be carried out at a high temperature (i.e. 50-100° C.), and the texturing component isopropyl alcohol is likely to volatilize toxic gases at a high temperature. Therefore, the method is not energy-efficient or environment-friendly, and is not suitable for popularization and application.
The invention entitled “Texturing method for diamond wire cut silicon slice” with the patent number 201110112185.1 discloses that texturing is carried out by adding an additive H2SO4 or H3PO4 which can control a reaction rate to a conventional acid texturing HF—HNO3 system. The invention entitled “Additive for acid texturing of diamond wire cut polycrystalline silicon slice and using method” with the application number 201310049552.7 discloses that a mixed aqueous solution of polyvinyl alcohol and polyethylene glycol as additives is added into a conventional acid texturing HF—HNO3 system. Both of the above patents can effectively reduce the reflectivity of silicon slices after texturing. However, due to insufficient reactivity and an insufficient degree of reflectivity reduction of a surface of diamond wire cut polycrystalline silicon, an efficient “light trapping” structure is hard to form on the surface of silicon slices, and the industry standard of photoelectric conversion cannot be met.
The invention entitled “Preparation method of RIE textured polycrystalline silicon solar cell” with the patent number 201410694985.2 discloses a preparation method of an RIE textured polycrystalline silicon solar cell. The so-called dry etching method is complicated in process. A damaged layer needs to be removed in a solution firstly, and then plasma generated by SF6 and Cl2 is used for etching in a high vacuum environment. The method can be used to prepare diamond wire cut textured polycrystalline silicon slices with low reflectivity, photoelectric conversion efficiency is improved, but equipment cost and equipment depreciation cost are high, and cost performance is poor. Besides, due to extra sulfide and halogen compound emissions, especially Cl2, a highly toxic gas, the environmental protection effect is insufficient.
In recent years, people have paid attention to a wet chemical method of etching polycrystalline silicon with assistance of metal ions to prepare a diamond wire cut polycrystalline silicon textured surface. Patent number 200780049395.2 discloses a method of etching a surface of polycrystalline silicon with assistance of metal ions represented by silver ions to prepare a diamond wire cut polycrystalline silicon textured surface. The method includes the following steps: putting a silicon slice in a mixed solution containing metal ions, an oxidant hydrogen peroxide and hydrofluoric acid for reaction to form a porous layer on the surface of the silicon slice, and then further etching the silicon slice with the porous layer in a mixed solution of hydrofluoric acid and nitric acid to form the textured surface with a large opening. The method can effectively reduce the reflectivity of the silicon slice and improve efficiency, but there are also obvious shortcomings: firstly, a uniform textured surface with small color difference and high light conversion efficiency cannot be formed on the surfaces of the polycrystalline silicon slices with various crystal planes such as [100], [110] and [111], i.e., the problem of difficult in achieving appearance uniformness of mass production cannot be solved; secondly, by putting the silicon slice in the mixed solution containing metal ions, the oxidant hydrogen peroxide and hydrofluoric acid for reaction to form the porous layer on the surface of the silicon slice, instability of the solution can be caused easily, the textured surface fluctuates greatly during batch reaction, and the service life of the solution is short. Patent number 201310127230.X discloses a textured structure of a crystalline silicon solar cell and a preparation method thereof. In the method, a layer of metal nanoparticles is coated on a surface, then the surface of a silicon slice is subjected to corrosion treatment, and finally the surface of the silicon slice is modified and etched. By means of the method, a textured polycrystalline silicon slice with low reflectivity can be obtained effectively, but the textured structure is limited to 100-500 nm, and the obtained textured slice also has the problems of textured surface nonuniformity and large color difference.
Therefore, there is an urgent need to research and develop a preparation method of a textured polycrystalline silicon slice which can improve the uniformity of a textured surface of a diamond wire cut polycrystalline silicon slice, increase surface reflectivity and realize high battery piece efficiency and is suitable for popularization and application.